The present invention relates to a method of manufacturing uranium dioxide fuel pellets. Particularly, this invention relates to a method of making U.sub.3 O.sub.8 single crystals and manufacturing large-grained uranium dioxide (UO.sub.2) fuel pellets through the use of a mixture comprising UO.sub.2 powder and U.sub.3 O.sub.8 single crystals.
Uranium dioxide (UO.sub.2) fuel pellets have been manufactured using UO.sub.2 powder according to the following processes; homogenizing UO.sub.2 powder or mixing UO.sub.2 powder with other additives, pressing UO.sub.2 powder into green pellets, and sintering the green pellets at about 1700.degree. C. in a reducing gas atmosphere to produce UO.sub.2 pellets. The UO.sub.2 pellet has a cylindrical shape of about 8 mm diameter and 10 mm length, and it has a density of about 95% of theoretical density (TD) and a grain size of about 8 .mu.m. UO.sub.2 pellets are loaded into a zirconium-based tube, which is then seal-welded to fabricate a fuel rod.
The defective UO.sub.2 pellets, which do not meet pellet specifications, are usually made in a small quantity during the process of pellet manufacture. Since defective UO.sub.2 pellets contain expensive enriched uranium, they are commonly recycled in the manufacture of new UO.sub.2 pellets according to the following method. Firstly, U.sub.3 O.sub.8 powder is made by heating defective UO.sub.2 pellets at around 450.degree. C. in air so as to oxidize UO.sub.2 to U.sub.3 O.sub.8, and the U.sub.3 O.sub.8 powder is then mixed with UO.sub.2 powder. Secondly, the mixture of UO.sub.2 and U.sub.3 O.sub.8 powder is pressed and sintered to produce UO.sub.2 pellets in the same way as the single UO.sub.2 powder. The U.sub.3 O.sub.8 powder is much less sinterable (capable of getting a high pellet density) than the UO.sub.2 powder, so that its content is generally limited within 15% by weight of the mixture of UO.sub.2 and U.sub.3 O.sub.8 powder. The art to improve the sinterability of U.sub.3 O.sub.8 powder has been disclosed in the literatures of U.S. Pat. No. 3,140,151 and U.S. Pat. No. 3,578,419.
While a fuel rod is irradiated (burned) in a nuclear reactor, the fission gas such as xenon and krypton is generated in a fuel pellet and is released to the outside of the fuel pellet. The pressure in a fuel rod builds up increasingly with burnup. The fission gas released should be maintained as low as possible, for an excessive pressure gives rise to the failure of a fuel rod. In high burnup, the fission gas released may restrict the performance of a fuel rod.
It has been known that a fuel pellet having a large grain provides a good performance since the amount of the fission gas released during irradiation decreases with increasing the grain size of a fuel pellet. It is a common sense that increasing sintering temperature and time makes a large-grained fuel pellet, but such a sintering method is not economical. Therefore, the art has been disclosed which provides a method of manufacturing large-grained UO.sub.2 pellets with the aid of sintering additives.
According to the method of U. S. Pat. No. 4,869,867, UO.sub.2 fuel pellets having an average grain size of at least 20 .mu.m are produced by adding aluminosilicate to UO.sub.2 powder, pressing and sintering. A shortcoming of the prior art is that the sintering additive may have an adverse effect on the other properties of the UO.sub.2 fuel pellet. For example, the sintering additive may degrade thermal properties such as thermal conductivity and melting point.
On the other hand, the art has been known which provides a method of manufacturing a large-grained UO.sub.2 pellet with no sintering additive added. U.S. Pat. No. 4,578,229 disclosed a method of sintering UO.sub.2 green pellets at a temperature range of 1000.degree. C. to 1400.degree. C. in an oxidizing gas such as carbon dioxide and reducing the sintered pellet at that temperature range in a reducing gas. The prior art has a problem in that a special sintering furnace is needed in which an oxidizing gas and a reducing gas can be used separately.